Conventional photography is based on the exposure of a film coated with a light sensitive emulsion. While this system has been highly refined over the years, it has several problems. First, film based systems are environmentally objectionable. The systems involve noxious chemicals such as silver and chemical developers whose disposal in an environmentally acceptable manner is becoming increasingly more costly.
Second, the film cannot be reused. Most photographers take several pictures for each picture that is actually kept. This leads to large numbers of negatives that are thrown away. In addition to the cost of the unused negatives, this practice further aggravates the above mentioned disposal problems.
Third, film has a finite storage life. This increases the cost of photography by requiring refrigerated storage and/or replacement for film that has passed its usable life.
Fourth, the dynamic range of film is less than adequate for many applications. Even black and white film has a gray scale of only 2.5-3 orders of magnitude. Color film is even more limited. In many applications, the range of intensities that must be recorded greatly exceeds this dynamic range. In such situations, at least some portion of the photograph must be over or under exposed.
Finally, correction of artifacts in photographs is difficult in film based systems. Altering the color of limited regions of a negative is all but impossible. Hence, artifacts such as "red eyes" in portraits taken with flash cameras must be handled by using special camera arrangements or by touching-up the prints. The latter approach requires talents not normally possessed by the average photographer.
These disadvantages together with the increased availability of low-cost computing systems have generated interest in solid state imaging systems such as CCD cameras and the like. Such cameras store their images on computer readable media such as magnetic disks. Since the image is computer readable, the image may be altered with the aid of a conventional computer workstation. Furthermore, these systems are environmentally superior to film in that they do not use noxious chemicals and the storage medium is reusable. Finally, solid state systems can have significantly more dynamic range than conventional film.
Unfortunately, solid state cameras having resolutions equivalent to the resolution available with photographic film are far too expensive for use by the average camera user. These systems are currently priced at 100 times the cost of an inexpensive camera. In addition, the user who is not computer literate has difficulty in having his or her images converted to conventional photographic prints.
Accordingly, there has been some interest in developing an alternative to film. Ideally, this alternative can be used in a conventional camera in place of conventional photographic film. For example, U.S. Pat. No. 5,065,023 to Lindmayer describes a material that utilizes electron trapping to store an image. An image projected on the surface of this material causes electrons to be elevated into the conduction band of the solid state material. The material is doped to have electron traps. The elevated electrons are trapped in spatially nearby traps. The density distribution of the trapped electrons in the material reflects the light intensity distribution of the incident image. This latent image is then read-out electro-optically by scanning the material with an infra-red beam that releases the electrons from the traps and produces visible light when the electrons re-enter their original energy states. The visible light generated by the recombination can be measured and recorded to reveal the original image.
To generate the equivalent of color film, the system taught by Lindmayer utilizes a three layer structure. Each layer consists of a solid state material having two dopants. The first dopant determines the color sensitivity of the layer, i.e., the color of light that will lift an electron into the conduction band of the crystal. The second dopant, which is the same for all layers, determines the energy level of the electron trap. The second dopant determines the wavelength of the light to be used in interrogating the material.
When the three layer structure is scanned with an infra-red beam, each layer emits light of a different color with an intensity that depends on the prior exposure of the film to light in a wavelength range determined by the first dopants. In general, the light emitted on scanning will be at different wavelengths than the incident light to which the first dopants were sensitive; however, a correct color image can be generated from calibration data and a knowledge of the dopants.
The preferred system taught by Lindmayer has several drawbacks. First, the system uses a multi-layer structure. To provide spatial resolution that approximates that of conventional film, the material must be deposited on a non-flat surface. The preferred surface may be viewed as being densely covered with optically isolated "pits" that are filled with the light sensitive material. The width of the pits determines the spatial resolution of the film, since they confines scatter from the light sensitive material to within each pixel. The depth of the pits relates to the quantum efficiency of the film.
There is no practical method for providing a three layer structure in such pits in which the layers are uniform in the amount of material in each layer. If the material quantities are not controlled, color and sensitivity distortions result.
Second, the system taught by Lindmayer requires three deposition steps to generate the three layers. This increases the cost of producing the film.
Lindmayer suggests that a single composition having all three particle types might be possible. In a co-pending patent application U.S. Ser. No. 08/338,922 to Cutler, an improved film composition is described in which the film alternative includes first, second, and third particle types in a common binder material. Each particle type comprises a crystalline base material having a trap dopant and a color dopant deposited therein. Each of the color dopants has a different activation energy for releasing electrons into the conduction/communication band of the crystalline base material.
While this improved composition eliminates the problems associated with the multi-layer structure of Lindmayer, the sensitivity of the color dopants in the red portion of the spectrum is insufficient to provide a film alternative with an acceptable ASA rating. Acceptable color dopants are known for the green and blue wavelengths. These color dopants have broad spectral peaks; hence, in principle, the tail of the green dopant can provide sensitivity in the red. However, the sensitivity provided by this approach is insufficient to provide an acceptable ASA rating for the film alternative.
Broadly, it is the object of the present invention to provide an improved color dopant for use in photographic film alternatives of the type described above.
It is a further object of the present invention to provide a color dopant that has greater sensitivity in the red portion of the spectrum than the dopants discussed for use in film alternatives of the type described above.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.